Ship Stability Criteria Calculator and Check

Introduction

This tool serves the purpose of computing a ship's righting arm (GZ-curve) to assess compliance with selected International Maritime Organization (IMO) stability criteria. To perform this calculation, you will need to input the cross curves of stability (KN) and the vertical center of gravity corrected for free surfaces. While not mandatory, you can also include the downflooding angle, which impacts the tool's results only if it falls below 40 degrees.

The calculation of GM (metacentric height) is estimated based on the KN input for 5 degrees. This estimate is valid when the GZ curve remains linear between 0 and 5 degrees. For a more accurate determination, you may use the formula GM = KMT - KG.

Example

Intact Stability Criteria Evaluation

This calculator examines the following IMO intact stability criteria:

  • The area under the GZ-curve must not fall below:
    • 0.055m×rad up to 30 degrees
    • 0.090m×rad up to 40 degrees
    • 0.030m×rad between 30 and 40 degrees
  • The Righting Arm (GZ) should exceed 0.20m.
  • The maximum righting arm (GZ) should occur at an angle greater than 30 degrees.
  • The minimum metacentric height (GMT) must be at least 0.15m

Abbreviations

  • KGcorrected - Vertical center of gravity measured from the baseline (top of keel plate) and corrected for the free surface effect. If unknown, you can compute it using the Loading Condition Calculator.
  • θf - Downflooding angle in cases of unprotected openings. Not mandatory.
  • KN - Cross Curves of Stability. Use the cubic interpolation calculator if some values are missing
  • θ - Angle of heel
  • GZ - Righting arm

Calculator

Loading condition data

Description Symbol Value Unit
Vertical center of gravity corrected for free surface effect KGcorrected m
Downflooding angle. Will only affect the results if less than 40 degrees. Keep empty if not known. θf deg

Cross Curves of Stability

θ [deg] KN [m]
5
10
15
20
25
30
35
40